Radiolabeled somatostatin analog, .sup.111 In-DTPA-octreotide, this new scintigraphic technique has attracted great interest in clinical nuclear medicine. The radiopharmaceutical has been used successfully for the localization of primary and metastatic somatostatin receptor-rich tumors, such as carcinoid, islet cell tumors of the pancreas, paragangliomas and small-cell carcinomas of the lungs. Octreotide comprises 8 amino acids which has the following structural formula: ##STR2## wherein the sulfur atoms of the Cys at the position 2 and of the Cys at the position 7 are mono-cyclic to form an --S--S-- bridge. When conjugated to the chelate diethylenetriaminepentaacetic acid (DTPA) and labeled with .sup.111 In, The product of .sup.111 In-DTPA-octreotide is a useful single-photon emission computed tomographic(SPECT) imaging agent for tumors containing somatostatin receptors. Synthesis of DTPA-octreotide can be carried out by two methods. The first method is synthesized initially by fragment condensation solution-phase procedures for octreotide. The synthetic process of octreotide has been described by Baner et al. in U.S. Pat. No. 4,395,403 in 1983. The process comprises:
&lt;i&gt;removing protected group from peptide; PA1 &lt;ii&gt;linking together by an amide bond two peptide unit; PA1 &lt;iii&gt;converting a function group at the N- or C-terminal; PA1 &lt;iv&gt;oxidizing a straight chain polypeptide by boron-tristrifluoroacetate. PA1 Fmoc: 9-fluorenylmethoxycarbonyl PA1 Boc: t-butyloxycarbonyl PA1 tBu: tert-butyl PA1 Trt: triphenylmethyl PA1 Thr(ol): threoninol residue PA1 Phe: phenylalanine residue PA1 Cys: cysteine residue PA1 Thr: threonine residue PA1 Lys: lysine residue PA1 Trp: tryptophan residue PA1 TFA: trifluoroacetic acid PA1 EDT: 1,2-ethanedithiol PA1 DTPA: diethylenetriaminepentaacetic acid PA1 DOTA: 1,4,7,10-tetraazacyclododecane-N,N',N",N'"-tetraacetic acid PA1 DMSO: dimethylsulfoxide PA1 (Boc).sub.2 O: di-tert-butyldicarbonate PA1 DIEA; N-ethyl-diisopropylamine PA1 PBS: phosphate buffer saline PA1 DMF: N,N-dimethylformamide PA1 BCA: bifunctional chelating agent PA1 THF: tetrahydrofuran PA1 HBTU: 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate! PA1 DIEA: N,N-diisoprepylethylamine
This process involves a time-consuming, multi-step synthesis, and it is difficult to separate octreotide from the reaction mixtures since all the synthesis steps are carried out in liquid phase.
DTPA-octreotide was synthesized by protecting the N.sup..epsilon. -amine group of the lysine residue of octreotide with a Boc group, followed by condensation of the unprotected N.sup..alpha. -amine group with DTPA.
The second method is synthesized by solid-phase procedure for DTPA-octreotide (J. Med. Chem. 1994, 37, 3749-3757). The procedure reacted DTPA with protected octreotide precursor on resin, before aminolysis with threoninol, followed by deprotection of Boc groups of D-Trp.sup.4, Lys.sup.5, and tBu group of Thr with trifluoroacetic acid (TFA). The overall synthetic yield of DTPA-octreotide by this protocol was 5%. The present invention provides a solid-phase synthesis of octreotide using 9-fluorenylmethoxycarbonyl(Fmoc) methodology. Octreotide was conjugated with bifunctional chelating agent in solution-phase.